Prior parallel-plate actuators, such as the example illustrated in FIGS. 1A (top view), 1B (side view), and 1C (side view), are typically designed with gaps 13 that are significantly larger than the stroke range of the actuator. When a voltage is applied between two electrode plates 15 and 10, an attractive force is produced between the electrode plates that rotates plate 10. Because the maximum rotation is determined by the separation, or gap 13, between the two electrode plates 15 and 10, there must be a large separation in order to obtain a large deflection. The gap 13 needs to be much larger than absolutely necessary for the physical movement of electrode plates 15 and 10, because if the electrodes approach too closely to each other (e.g., less than about ⅓ of gap 13), a point of instability is reached where the electrodes 15 and 10 may snap together.
Because the force produced by a parallel-plate actuator is proportional to (voltage/gap)2, as gap 13 increases, the voltage must also go up with the square of the distance in order to achieve the same force. With the movement of the structure, electrode plates 15 and 10 do not remain parallel to each other and gap 13 between them decreases. Hence, the voltage required to move electrode plates 15 and 10 a given distance is high, nonlinear, and constantly changing. This may require more complex electronics to control the actuator that may be difficult and costly to build. Also, the use of a large gap may result in cross-talk between adjacent actuators in an array.
Moreover, on the extremely small scale of these actuators, problems are introduced by the need to run conductors for the voltages very close together. With higher voltages, interactions between conductors are hard to avoid and in extreme cases, arcing between conductors will occur, leading to damage to the device. Current parallel plate actuators having a useful range of movement typically require voltages of 300 volts or higher.
U.S. Pat. No. 5,536,988 entitled Compound Stage MEM Actuator Suspended For Multidimensional Motion discloses the use of interlocking comb fingers as X-Y axis actuators for nested stages of MEMs devices. The levitation force produced by comb fingers can also be use to generate torsional actuators. Nevertheless, the primary limitation of comb fingers is on the stroke range. The levitation force produced by comb drives is limited to approximately the same distance that the comb fingers are spaced. This typically makes deflections greater that 5 to 10 microns (μm) very difficult. Deflections greater than 50 μm may be needed, however, for mirror actuator applications, which may not be possible to achieve with the comb finger actuators.